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 A070746 Numbers k such that k 1's followed by k is a prime. 10
 1, 7, 709, 2203, 4481, 5107, 11489 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS The number corresponding to a(3) has been certified prime with Primo; those corresponding to a(4), a(5), a(6) are probable primes. a(8) > 147000, if it exists. - Giovanni Resta, Jun 22 2018 LINKS EXAMPLE 7 is in the sequence because 11111117 (seven 1's followed by 7) is prime. MATHEMATICA Do[ If[ PrimeQ[ FromDigits[ Join[ IntegerDigits[(10^n - 1)/9], IntegerDigits[n]]]], Print[n]], {n, 1, 2000, 2}] PROG (WinPFGW) Primality testing r(5107)*10^len(5107)+5107 [N-1/N+1, Brillhart-Lehmer-Selfridge] r(5107)*10^len(5107)+5107 is Fermat and Lucas PRP! (79 seconds). - Jason Earls (PARI) isok(n) = isprime(eval(concat(apply(x->Str(x), vector(n+1, j, if (j>n, n, 1)))))) \\ Michel Marcus, Jun 15 2018 CROSSREFS Cf. A068817, A084428. Sequence in context: A129291 A229149 A159815 * A068004 A098110 A062334 Adjacent sequences:  A070743 A070744 A070745 * A070747 A070748 A070749 KEYWORD base,nonn,more AUTHOR Robert G. Wilson v, May 03 2002 EXTENSIONS a(4) from Jason Earls, May 06 2002 a(5) from Rick L. Shepherd, Sep 02 2002 a(6) from Farideh Firoozbakht, Jun 29 2003 a(7) from Farideh Firoozbakht, Jul 04 2003 Edited by T. D. Noe, Oct 30 2008 STATUS approved

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Last modified June 15 04:47 EDT 2021. Contains 345043 sequences. (Running on oeis4.)